Magnetic sensors operate on the principle of detecting magnetic flux density modulation caused by the movement of appropriately configured reluctors (or targets). The magnetic sensor must be affixed very close to the reluctor since its sensitivity decreases very rapidly with the size of the air gap between the reluctor and the magnetic sensor. In most automotive applications, for example, the air gaps are on the order of 0.3 to 1.75 mm. Over such a range of air gaps, the sensor output signal decreases more than ten times. The signal attenuation at large air gaps makes the sensor operation more prone to noise induced failures as well as less accurate in detecting the elements of the reluctor as it spins in relation to the magnetic sensor. Both of these factors are often unacceptable in critical engine control and diagnostic applications.
It may at first glance appear that there would be no problem whatsoever to choose and achieve an appropriate air gap between the magnetic sensor and the reluctor. However, in the majority of production cases, the stack-up of tolerances of the many different components randomly cause internal air gaps that influence the net size of the air gap, which consequently precludes achieving, at each assembly, a precisely predetermined external air gap between the magnetic sensor and the reluctor by mere assembly of the parts. As a result, because of the random variations caused by accumulation of tolerances, mere assembly of the parts risks damaging interference between the magnetic sensor and reluctor on the one hand, and inaccurate readings associated with too large a net air gap on the other hand. To lessen all the tolerances so that mere assembly assures, at each assembly, the optimum external air gap is physically difficult to obtain and involves costs associated with manufacturing such precise parts.
The majority of magnetic sensors used in automotive applications involve non-adjustable air gap placement, wherein the stack-up of tolerances results in an internal air gap that causes deviation from the optimal external air gap. For example, a rigid bracket is affixed to the body of a magnetic sensor. The magnetic sensor is placed into a sensor bore in the engine block, and the bracket is bolted, via a bolt hole in the bracket, to a threaded mounting hole in a mounting surface of the engine block. When the bracket is bolted, the length of the sensor body from the bolt hole of the bracket to the sensor tip determines the external air gap with respect to the reluctor, which air gap is affected by the stack-up of tolerances. Even though subject to tolerance related placement inaccuracy, this structural mounting methodology is used widely because of the simplicity of the hardware, and ease of assembly and service.
In situations where external air gap variation cannot be tolerated, the external air gap is preset during magnetic sensor installation by means of an adjustable bracket, often referred to as a “side-mount” bracket. The adjustability of side-mount brackets resides in a bolt slot which allows for the bracket to be adjusted along the slot elongation relative to the threaded mounting hole of the mounting surface.
In one form of operation of the side-mount bracket, the sensor body is placed into the sensor bore of the engine block such that the sensor tip is allowed to touch the surface of the reluctor, and then it is withdrawn a distance equal to the predetermined optimum external air gap. This method is time consuming.
In another form of operation of the side-mount bracket, a gauging layer of soft, abradable material is placed onto the sensor tip, wherein the thickness of the gauging layer is equal to the optimum external air gap. The gauging layer may be either attached to the sensor body or be a part thereof, such as a protuberance, provided the sensor body is of a soft material. Now, the installer need merely place the sensor body into the sensor bore until the gauging layer touches the reluctor, and then tighten the bolt on the mounting surface to thereby hold the sensor body at this position. During initial rotation of the reluctor, a portion of the gauging layer is sacrificial to abrasion due to reluctor runout or differential thermal expansion without damage being incurred to the sensor body or the reluctor.
In the event the magnetic sensor must be re-installed, an abraded gauging layer cannot again provide position location for the sensor tip, as it was formerly able to do when it was unabraded. Therefore, before dismounting the magnetic sensor, the bracket must be marked to indicate the correct position of the sensor body relative to the bracket so that when the new magnetic sensor is re-installed, its position on the bracket can be alignably sighted—not an exact procedure. Indeed, rather than try to reinstall the old, but still usable, sensor using the sighting method to reset the external air gap, a technician would rather install a new sensor having the abradable layer intact, thereby circumventing the sighting step otherwise needed to reinstall the old, but usable, sensor. This results in waste of otherwise good sensors and unnecessary expense for the customer or warranty provider.
In the prior art, it is known to precisely adjust the external air gap using a threaded sensor body housing and threaded sensor bore. This structure is generally used exclusively with magnetic sensors having a single sensing element and having sensing capability unaffected by sensor rotation around its longitudinal axis. In this approach, the housing bottom is brought into touching engagement with the reluctor, and then the sensor body housing is rotated a predetermined angular amount, wherein the pitch angle of the threads raises the housing bottom a distance equal to the optimum external air gap. However, the sensor must then be inserted in the housing making sure that the sensor tip is bottomed out against the inside housing bottom to maintain the set external air gap between the outside bottom of the housing and reluctor. Otherwise, an internal air gap is created reducing the effectiveness of the sensor by altering the net air gap between the sensor tip and the reluctor. The prior art has alleviated problems associated with internal air gaps by heat staking or ultrasonic joining and then biasing the sensor tip to maintain contact with the housing bottom. These approaches prove costly and timely, as well as lacking in longevity.
Accordingly, what is needed in the art is a method and apparatus for assembling a magnetic sensor which is easy and cost effective to install, and provides for automatic setting of an optimal external air gap, while eliminating any internal air gap caused by stack-up tolerances during assembly of the sensor assembly.